An imaging device, or sensor, is a photosensitive electronic component that converts electromagnetic radiation into an analog electrical signal. This signal is thereafter amplified and then digitized by an analog-digital converter and finally processed to obtain a digital image.
The imaging device exploits the photoelectric effect, which allows the incident photons to tear electrons away at each active element, called a photosite (pixel). An imaging device generally comprises photosites arranged in a matrix, each photosite corresponding to a pixel of an image. The photons sensed by the imaging device based on semi-conductor components are converted into photocarriers in the silicon, i.e. into electron/hole pairs. More precisely, the charge created in the photosensitive zones is stored in the photosite before being read by an electronic system.
Two main families of imaging devices, or sensors, are available: charge transfer sensors, i.e. Charge-Coupled Device (CCD) sensors, and complementary metal-oxide semiconductors (CMOS) sensors, or CMOS Active Pixel Sensor (APS) sensors. A typical photosite generally comprises a photodiode intended to transform the sensed photons into a corresponding electrical signal by way of the photoelectric effect. Accordingly, the photodiode generally comprises a PN junction formed in a semi-conductor substrate.
Two types of photodiodes exist: photodiodes comprising solely a PN junction and pinched photodiodes comprising an N-doped layer lying between two P-doped layers, the N-doped layer being optimized so as to be completely deserted of free carriers after the charge transfer transistor control cycle. A PNP pinched photodiode differs from a photodiode comprising a simple PN junction in that it is coupled to a charge transfer gate, making it possible to control the charge transfer to the reading node, on the one hand, and in that the N-doped layer is deserted of any charge after the transfer. A PNP pinched photodiode thus allows correlated double sampling, i.e. sampling comprising on each occasion a measurement without charge followed by a measurement with charge.
Pinched photodiodes operate by accumulation of charge in a charge storage zone, then transfer of the charge into the capacitor of the reading node, and then measurements of the voltage delivered by this reading node by virtue of a follower transistor. Pinched photodiodes thus make it possible to accumulate a quantity of charge proportional to the luminous signal and to an integration time.
A pinched photodiode comprises a charge collection zone defined by the three layers respectively P1, N and P2 doped, and a charge storage zone comprising the N-doped layer. Under charge accumulation conditions, a turn-off potential is applied to the gate of the charge transfer transistor. A bias potential, for example, 0V, is also applied to the P1-doped and P2-doped layers. This bias potential for the P1-doped and P2-doped layers makes it possible to define a reference potential for the charge storage zone, i.e. the N-doped layer. The turn-off potential and the reference potential of the charge storage zone make it possible to define the boundary conditions of a potential well, which characterize the charge storage zone. The potential well thus defined depends not only on the value of the two potentials, but also on the value of the doping of the N-doped layer and the geometric dimensions of the photodiode.
Indeed, the dimensions of this potential well, especially its maximum depth corresponding to the depletion potential denoted Vmax, i.e. the voltage of the potential well when it is empty of charge, are defined principally by the value of the effective doping (“Net doping”) of the zones P1, P2 and N of the photodiode and by its geometric dimensions corresponding to its width, length and thickness. The thickness corresponds to the distance between the two junctions P1N and NP2.
The charge storage capacity of the photodiode, denoted Qsat, depends on the dimensions of the potential well, especially the depletion potential and the volume of the storage zone. The depletion potential corresponds to the potential difference between the potential of the N layer where the potential well is maximized and the bias potential for the P1 and P2 layers when the N layer is completely deserted of free carriers.
The quality of the image obtained is defined, on the one hand, by the charge storage capacity within the potential well of the storage zone. Indeed, the electrical signal generated depends on the amount of charge stored and then transferred. But the quality of the image obtained is defined, on the other hand, by the charge transfer between the charge storage zone and the reading node via the charge transfer transistor.
Indeed, to obtain good image quality, it may be necessary that all the charge accumulated in the charge storage zone be transferred. If not all the charge is transferred for each of the photosites, there is a risk of electron noise in the image and a remnant effect, which may be present in the following image.
For example, two adjacent photosites that have received identical photons might not produce exactly the same image on account of the existence of residual charge in the photosites on completion of the charge transfer, and might therefore impair the quality of the image obtained. Moreover, in the case of two successively captured identical images, the latter might differ if the charge transfer is not total in each photosite.
Finally in a case where charge remains in the photodiode after the transfer, this charge might modify the following image, causing a remnant effect. The degree of transfer therefore directly affects the quality of the image obtained, via a disturbance of the electrical signal generated.
In a PNP pinched photodiode, the maximum quantity of charge stored in the charge storage zone varies with the depletion potential Vmax, whereas the optimization of the charge transfer varies inversely with the depletion potential Vmax. Indeed, to increase the charge storage capacity, Qsat, the depletion potential Vmax may be increased, whereas to improve the charge transfer, the depletion potential Vmax may be decreased.
U.S. Patent Application Publication No. 2010/0176276 to Ihara discloses a photosite comprising a pinched photodiode and a charge transfer transistor whose shape has been modified so as to adopt a general T-shape. The T-shaped transfer transistor thus makes it possible to take the transistor above the zone of maximum charge storage, i.e. to the level of the charge storage zone where the potential corresponds to the depletion potential.
However, a configuration comprising a transfer transistor having a peninsular part going above the photodiode may have several drawbacks. On the one hand, a part of the photodiode is covered by the peninsular part of the charge transfer transistor and the area of the photon capture surface is consequently reduced. Indeed, the peninsular part of the charge transfer transistor situated above the photodiode, reflects and absorbs a considerable quantity of photons irradiating the photodiode.
Moreover, such a configuration may no longer make it possible to carry out a self-alignment of the photodiode with respect to the charge transfer transistor. Indeed, in such a configuration, the charge transfer transistor may be placed after the implantation of the photodiode in such a way that the peninsular part is positioned above the zone of maximum charge storage where the potential well is at its maximum. Whereas, in a conventional photosite production process, the photodiode is implanted after the transistor, the photodiode then aligned with respect to the transfer transistor.